Thermal Barrier Coatings (TBCs) generally comprise a two-layer system, which incorporates an outer insulative ceramic layer and an underlying oxidation-resistant metallic layer (bond coat, referred to by some in the field as a basecoat) on an external surface of metallic components. Typically, the bond coat of such TBC system itself represents a complex chemical system, identified in the art as M-Cr—Al—X, where M is nickel, cobalt or a combination of both and X is usually yttrium, but alternatively may be Si, Ta, or Hf. The alloy composition of the bond coat is selected to provide a best fit for oxidation and corrosion resistance. While nickel base alloys generally demonstrate better oxidation resistance, the cobalt base alloys provide better hot corrosion resistance. In general, these bond coats are deposited using air plasma or vacuum plasma/low pressure plasma, wire-arc, flame combustion, high velocity oxy-fuel or cold spray process, depending on operational and other requirements.
For example, a TBC system may utilize a ceramic top coat such as yttria stabilized zirconia, that is applied over the bond coat. Such ceramic top coat is typically applied by either electron beam physical vapor deposition (EB-PVD) or by plasma spray. Prior to ceramic top coat application, the surface of the bond coat is optimized to maximize adherence between the bond coat and the specific ceramic top coat used. For EB-PVD, the bond coat is usually polished and preoxidized prior to deposition of a columnar ceramic top coat, which provides a thermal barrier. In contrast, plasma sprayed ceramic top coats favor a rough bond coat surface and do not require the in-situ formation of an aluminum oxide layer prior to deposition. Plasma sprayed ceramic thermal barrier coatings rely on porosity and micro cracks to accommodate strain during service.
FIG. 1A shows a cross-sectional side view schematic of a prior art conventional TBC system 100. Conventional TBC system 100 is comprised of a substrate 120, a bond coat 130, and an outer ceramic layer 140. FIG. 1B provides a cross-sectional view of an actual prior art TBC system 100 formed with a convention bond coat chemistry, where a top portion of substrate 120 is observable, above which is disposed bond coat 130, above which is disposed outer ceramic layer 140.
Generally, upon high temperature exposure in operation, the bond coats grow a surface protective oxide layer, due to the selective oxidation of the elemental constituents in the bond coat alloy. The desired properties of this oxide layer are thermodynamic stability, slow growth and adherence. Currently, the majority of the alloys are chromia or alumina formers and the presence of relatively low concentrations of yttrium in the bond coat improves the adherence of the oxide layer formed at the bond coat/ceramic top coat boundary to the ceramic top coat. However, the oxide scale possess high thermal conductivity and low thermal expansion compared to the overall system (and particularly to the ceramic layer 140), thus increasing the residual stress at the oxide/ceramic layer interface leading to spallation.
FIG. 2 shows one such example of delamination of a prior art TBC system 200. The delamination regions 210 are observable between bond coat 230, at the more exterior edge of which is an Al2O3 layer 233, and overlaying ceramic layer 240. An upper portion of substrate 220 also is observable.
Currently, a vast amount of research is directed towards formation of continuous and more adherent surface oxide layer. For example, it is known that addition of small amounts of reactive elements promote this formation, where the alloying additions are Cr, Si, Ta, Hf and precious metals (Pt, Pd). It also has been taught, in U.S. Pat. No. 5,993,980, issued Nov. 30, 1999 to Schmitz and Czech, that certain formulations of an adhesion-promoting layer may include from 0.3 to 2.0 percent yttrium and/or at least one equivalent metal from the group including scandium and rare earth elements. An earlier reference, PCT Publication number WO/89/07159, cited in U.S. Pat. No. 5,993,980, is stated to disclose that an outer alloy of a two-layer metallic protective coating may comprise between 0.2 and 3.0 percent of at least one element from a list including yttrium and other metals, however excluding other rare earth metals.
Despite these and other formulations and approaches toward developing better TBC systems, a need remains for TBC formulations and systems that are directed to solving persistent problems with TBC systems, such as extending component life.